Plug-in
Definition
Software that extends the functionality of a DAW by adding virtual instruments, effects processors, or utility tools. Common formats include VST3, AU, and AAX.
In Simple Terms
Software that adds instruments or effects to your DAW—compressors, reverbs, synthesizers, guitar amps. They extend what your DAW can do, like apps extend what your phone can do.
In Practice
A compressor plug-in inserted on a vocal channel processes the signal in real time within the DAW. Industry standards like the FabFilter Pro-C2 or Universal Audio SSL G-Bus emulation are common choices.